The evolution of the electronics industry in recent years has resulted in a steady decrease of integrated circuit feature sizes. With the introduction of the 256K chip and research on the 512 K chip in progress, feature sizes are shrinking to such a degree that many of the traditional semiconductor processing techniques are no longer adequate. Until recently, doped polysilicon has been used extensively as a conductor for gates and gate interconnects on metal-oxide semiconductor (MOS) devices. Doped polysilicon was chosen because it can withstand subsequent high temperature processing steps and because it has electrical properties, such as a bulk resistivity of about 1,000 .mu..OMEGA.-cm, which are desirable. As conductor line widths are reduced to below 2.mu., however, the resistance of polysilicon conductive lines is large enough to degrade the high speed performance of devices. Thus, with minimum feature sizes of 1.mu. or less, the electronics industry has looked to refractory metal silicides as a solution to gate and gate interconnect problems in high density chip production.
Refractory metal silicides are now being used in place of polysilicon or in addition to polysilicon (as a two layer polysilicon-silicide conductor sometimes referred to as a polycide). Refractory metal silicides have very low bulk resistivities (approx. 15-100 .mu..OMEGA.-cm), can withstand temperatures in excess of 1,000.degree. C. and, in general, do not oxidize easily. The silicides commonly associated with the formation of gate interconnects are titanium silicide (TiSi.sub.2), tungsten silicide (WSi.sub.2), molybdenum silicide (MoSi.sub.2) and tantalum silicide (TaSi.sub.2).
The term metal silicide as used herein is defined as a metallic compound having the general formula M.sub.x Si.sub.y where M=a metal, 0&lt;x.ltoreq.1 and 0&lt;y.ltoreq.1.
Currently, a variety of methods are employed to produce conductive silicide coatings. They include sputtering or co-sputtering techniques, evaporation or co-evaporation processes, chemical vapor deposition processes requiring high substrate temperatures (pre- or post-deposition), and plasma induced chemical vapor deposition. For a detailed discussion of many of these methods see Murarka, Refractory Silicides for VSLI Production, Academic Press, 1983, pp. 115-31.
Sputtering techniques employ target materials which are bombarded by energized ions to free atoms of the material for deposition onto a substrate. The techniques include sputtering of a metal or silicon or polysilicon or simultaneously sputtering from two targets (co-sputtering). Both techniques employ radio frequency (RF, plasma) or direct current (DC) magnetron sputtering for silicide formation.
DC magnetron sputtering is of limited use because it requires that the target material be a conductor. Thus, when a metal target and a silicon target are employed in a co-sputtering process, the silicon target must be doped to make it a good conductor. Consequently, the resultant silicide layer may include an unwanted dopant.
Sputtering techniques cause gas entrapment and/or contamination to incur in the deposited layer. The bombarding ions are gas ions which become entrapped in the silicide layer. These gas ions can chemically contaminate the layer if the gas contains a chemically reactive impurity.
Silicide coatings produced by the physical deposition process of sputtering or co-sputtering are invariably amorphous. These amorphous coatings have a high resistivity as deposited and must be annealed at 900.degree. C.-1,100.degree. C. for 0.5-1.0 hr. to produce high quality coatings of low resistivity.
Poor step coverage is a drawback associated with sputtering or co-sputtering. Because the topography of the wafer surface is varied, good step coverage is necessary to avoid degradation or failure of the device. Sputtered and co-sputtered layers exhibit poor step coverage as compared to other prior art processes. The result of poor step coverage is localized thin spots which can produce overheating, electromigration of the conductor and, consequently, premature failure of the device.
Evaporation techniques include evaporation of metal on silicon or polysilicon and co-evaporation of metal and silicon on silicon, polysilicon, or oxides. The techniques use heat (resistive, inductive (RF), electron bombardment or laser) to vaporize the elements which are deposited on the substrate surface. One problem specifically associated with co-evaporation is consistency of the silicide composition from run to run. In addition, electron guns, commonly used as the heat source for refractory metal silicide formation, cause radiation damage to the substrate. Furthermore, step coverage by evaporation techniques is generally no better than the coverage produced by sputtering techniques.
Chemical vapor deposition (CVD) of silicides requires a chemical reaction of materials in the vapor phase or a reaction which occurs on the substrate surface. Chemical vapor deposition requires a high substrate temperature to produce a conductive coating (1,000.degree. C.-1,100.degree. C. for TiSi.sub.2, See Wahl et al., "The CVD Deposition of Ti-Si Containing Coatings on Ni-Base Superalloys," Proceedings of the Eighth International Conference on Chemical Vapor Deposition, Electrochemical Society, Pennington, N.J., 1981, pp. 685-98). Unlike the step coverage problems associated with sputtering or evaporation of silicide films, CVD films exhibit good step coverage.
Cold wall, low pressure CVD processes have developed quite recently as an alternative to high substrate temperature processes. Examples of these processes are described in "Cold Wall, Low Pressure CVD Reactor", Solid State Technology, Nov. 1983, pp. 63-4 and Brors et al., "Properties of Low Pressure CVD Tungsten Silicide as Related to IC Processing Requirements," Solid State Technology, Volume 26, No. 183, Apr. 1983, pp. 183-6. The product, produced on a substrate heated to a low temperature, is either microcrystalline or amorphous. In order to produce a conductive layer, a high temperature anneal (1,000.degree. C.-1,100.degree. C.) is required.
A plasma induced CVD process is briefly disclosed in "Plasma Titanium-Silicide . . . Path of Least Resistance," Solid State Technology, Jan. 1984, p. 37. A principle drawback of this process is the distinct possibility of radiation damage to the substrate.
All of the processes described supra require high capital expeditures and considerable maintenance expenses in order to be effectively operated. In addition to the economic drawbacks, these processes are difficult to control and, therefore, the reproducability of chemically consistant products is a significant problem.
Most recently, laser induced chemical vapor deposition (LCVD) has been used to produce semiconducting, insulating and conductive (metal) coatings. The laser induced reactions cause the gaseous constituents to react and produce a coating on the substrate. References discussing the production of Si semiconductive layers, oxide and nitrite insulating layers and conductive metal layers include: U.S. Pat. No. 4,227,907; U.S. Pat. No. 4,270,997; U.S. Pat. No. 4,260,649; U.S. Pat. No. 4,324,854; Bilenchi et al., "Laser-Enhanced Chemical Vapor Deposition of Silicon," Proceedings of the Eighth International Conference on Chemical Vapor Deposition, Electrochemical Society, Pennington, N.J., 1981, pp. 275-83; Gattuso et al., "IR Laser-Induced Deposition of Silicon Thin Films," Mat. Res. Soc. Proc., Volume 17, 1983, pp. 215-22; Allen et al., "Summary Abstract: Properties of Several Types of Films Deposited By Laser CVD," Vac. Sci. Technol., Mar. 1983; and, Meunier et al., "Hydrogenated Amorphous Silicon Produced by Laser Induced Chemical Vapor Deposition of Silane," Appl. Phys. Lett. 43 (3), Aug. 1, 1983, pp. 273-5. In addition, a comprehensive list of documented laser induced reactions is disclosed in Steinfeld, "Laser-Induced Chemical Reactions: Survey of the Literature, 1965-1979, Plenum:New York, 1981, pp. 243-67.
An excellent method for overcoming the problems associated with the prior art processes is disclosed in related U.S. Pat. No. 4,568,565 entitled "Light Induced Chemical Vapor Deposition of Conductive Titanium Silicide Films". That application describes light induced chemical vapor deposition processes for forming conductive metal silicide containing films at low substrate temperatures.
We have discovered a process for forming conductive films comprising metal silicides which is simple and easily monitored (in terms of coating thickness, step coverage, etc.), requires low capital expenditures and yields chemically consistent products.